Pneumatic servomotors such as disclosed in U.S. Pat. No. 3,106,873 are constructed in a manner such that air from the environment directly entered the rear chamber thereof by passing through the axial passageway in which the control valve is located. As air passed through the axial passageway into a vacuum evacuated rear chamber, a hissing noise can occur. This hissing sound does not affect the operational functioning of the servomotor but is annoying to some operators of a vehicle so equipped. However, it soon becomes evident that the operation of the control valve is affected by contaminants carried by the air. As a result a series of filters, such as shown in U.S. Pat. No. 3,289,547, were required to assure that only clean air was presented to the control valve. In addition, the filter, as disclosed in U.S. Pat. No. 3,897,716, reduced the air through the axial passageway to correspondingly reduce the hissing noise on normal brake application sufficiently that most operators do not hear or are not bothered by such operational noise.
However, some vehicle manufacturers as part of their sales and advertising procedure compare how quiet the passenger compartment is with respect to other vehicles. In an effort to further reduce the operational noise of a brake servomotor, it was suggested that the operational air be communicated from the engine compartment thereby eliminating the removal of air from the passenger compartment to operate the servomotor. In such servomotors, air is required to flow along the outside of the cylindrical housing of the hub and make a 180.degree. turn around the end thereof before entering the axial passage to be presented to the control valve. Unfortunately, because of the rapid flow of air around the end of the cylindrical housing, it is possible to cause the dust boot to flutter and create an undesirable noise.